Some people might have gotten the impression from reading WUs in this node that every Japanese car with car-company stickers on it is rice. This could not be farther from the truth. The term "Rice Boy" explicitly refers to the driver of a car that is a victim of "stickerformance" - the seeming idea that if you apply a bunch of performance-company stickers to your vehicle, it will be faster.
Contrary to popular belief, a lowering kit and a loud exhaust on a Japanese car is not necessarily a sign of a rice boy, although it is an excellent indicator. It is also not necessary for the car to be sold under a Japanese label; there are numerous examples of Fords and Chevrolets that are "riced out". These cars are however typically either based closely or loosely on American designs. Most of them are a Japanese car with American styling and interior, the primary example being the Ford Escort, which is a Mazda 323.
However, the big exhaust tip on the factory exhaust, and the lowering job that doesn't take camber into account such that the back wheels on a Honda Civic (the leading rice car) stick out like clown shoes are pretty clear signs of riceboydom, particularly when the car is also covered with stickers for companies whose parts don't even appear on the vehicle, or are perhaps represented only by a steering wheel or shift knob.
An even clearer sign is a big ugly wing or spoiler on the vehicle, especially if it is not otherwise modified. A wing with a useful amount of downforce is both very large, and utterly useless at speeds below about 100 mph. Another indicator is the "Altezza" tail lights, the type that have the clear lens covering a silver reflector and a red lens, but these are now being put on all types of vehicles by a broad assortment of people. Regardless, all of those people should take a hint from the name ("Altezzas") which indicates that they belong on the Altezza, and not on a Chevrolet pickup.
Rice boys themselves tend to feature similar characteristics, most of them centering around trying to look asian. This is true even when they are asian; in such a case they will dress and style their hair in such a way to accentuate a stereotype. There is, however, no particular guarantee thereof. In general they are quite young.
Rice boys are given to unnecessary celebration of so-called "victories" over other vehicles. In most cases, the other driver isn't even trying to race them, but simply has such a great advantage in power that just a casual "launch" from a stop will beat out an unmodified ricemobile. They are also known to pull extremely dangerous stunts in traffic for minimal speed gains, in particular weaving through traffic and passing on the right without waiting for someone to get over. They are also known to pull up behind fast-looking vehicles on highways and flash their brights at them, which means they are looking for a race.
One last thing to remember; as others point out, a little Japanese commuter car isn't necessarily a pushover. They are almost without exception extremely light in weight and generally have excellent suspension systems, aside from the "real" sports cars, which are often fairly heavy in order to promote stability. A base-model Civic with a bolt-on turbocharger can easily have a far superior power to weight ratio when compared to, for example, a Mustang GT, or more especially when compared to even larger "muscle cars." The current Mitsubishi Lancer Evolution VII has 300 horsepower in stock form with only a two-liter four-cylinder engine through the use of aggressive turbocharging. Even more power can be wrung out of two liters, with power levels over 400 horsepower fairly common.